Multi-mode fiber amplifier

ABSTRACT

A laser utilizes a cavity design which allows the stable generation of high peak power pulses from mode-locked multi-mode fiber lasers, greatly extending the peak power limits of conventional mode-locked single-mode fiber lasers. Mode-locking may be induced by insertion of a saturable absorber into the cavity and by inserting one or more mode-filters to ensure the oscillation of the fundamental mode in the multi-mode fiber. The probability of damage of the absorber may be minimized by the insertion of an additional semiconductor optical power limiter into the cavity.  
     To amplify and compress optical pulses in a multi-mode (MM) optical fiber, a single-mode is launched into the MM fiber by matching the modal profile of the fundamental mode of the MM fiber with a diffraction-limited optical mode at the launch end, The fundamental mode is preserved in the MM fiber by minimizing mode-coupling by using relatively short lengths of step-index MM fibers with a few hundred modes and by minimizing fiber perturbations. Doping is confined to the center of the fiber core to preferentially amplify the fundamental mode, to reduce amplified spontaneous emission and to allow gain-guiding of the fundamental mode. Gain-guiding allows for the design of systems with length-dependent and power-dependent diameters of the fundamental mode. To allow pumping with high-power laser diodes, a double-clad amplifier structure is employed. For applications in nonlinear pulse-compression, self phase modulation and dispersion in the optical fibers can be exploited. High-power optical pulses may be linearly compressed using bulk optics dispersive delay lines or by chirped fiber Bragg gratings written directly into the SM or MM optical fiber. High-power cw lasers operating in a single near-diffraction-limited mode may be constructed from MM fibers by incorporating effective mode-filters into the laser cavity. Regenerative fiber amplifiers may be constructed from MM fibers by careful control of the recirculating mode. Higher-power Q-switched fiber lasers may be constructed by exploiting the large energy stored in MM fiber amplifiers.

RELATED APPLICATIONS

[0001] This application is a continuation application of U.S.application Ser. No. 09/785,944 filed Feb. 16, 2001, which is acontinuation application of U.S. application Ser. No. 09/199,728 filedNov. 25, 1998, now U.S. Pat. No. 6,275,512 issued Aug. 14, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates to the amplification of single modelight pulses in multi-mode fiber amplifiers, and more particularly tothe use of multi-mode amplifying fibers to increase peak pulse power ina mode-locked laser pulse source used for generating ultra-short opticalpulses.

[0003] The present invention relates to the use of multi-mode fibers foramplification of laser light in a single-mode amplifier system.

BACKGROUND OF THE INVENTION Background Relating to Optical Amplifiers

[0004] Single-mode rare-earth-doped optical fiber amplifiers have beenwidely used for over a decade to provide diffraction-limited opticalamplification of optical pulses. Because single mode fiber amplifiersgenerate very low noise levels, do not induce modal dispersion, and arecompatible with single mode fiber optic transmission lines, they havebeen used almost exclusively in telecommunication applications.

[0005] The amplification of high peak-power pulses in adiffraction-limited optical beam in single-mode optical fiber amplifiersis generally limited by the small fiber core size that needs to beemployed to ensure single-mode operation of the fiber. In general theonset of nonlinearities such as self-phase modulation lead to severepulse distortions once the integral of the power level present insidethe fiber with the propagation length exceeds a certain limiting value.For a constant peak power P inside the fiber, the tolerable amount ofself-phase modulation Φ_(nl) is given by${\Phi_{nl} = {\frac{2\quad \pi \quad n_{2}{PL}}{\lambda \quad A} \leq 5}},$

[0006] where A is the area of the fundamental mode in the fiber, ë isthe operation wavelength, L is the fiber length and n₂=3.2×10⁻²⁹ m²/W isthe nonlinear refractive index in silica optical fibers.

[0007] As an alternative to single-mode amplifiers, amplification inmulti-mode optical fibers has been considered. However, in general,amplification experiments in multi-mode optical fibers have led tonon-diffraction-limited outputs as well as unacceptable pulse broadeningdue to modal dispersion, since the launch conditions into the multi-modeoptical fiber and mode-coupling in the multi-mode fiber have not beencontrolled.

[0008] Amplified spontaneous emission in a multi-mode fiber has beenreduced by selectively exciting active ions close to the center of thefiber core or by confining the active ions to the center of the fibercore. U.S. Pat. No. 5,187,759, hereby incorporated herein by reference.Since the overlap of the low-order modes in a multi-mode optical fiberis highest with the active ions close to the center of the fiber core,any amplified spontaneous emission will then also be predominantlygenerated in low-order modes of the multi-mode fiber. As a result, thetotal amount of amplified spontaneous emission can be reduced in themulti-mode fiber, since no amplified spontaneous emission is generatedin high-order modes.

[0009] As an alternative for obtaining high-power pulses, chirped pulseamplification with chirped fiber Bragg gratings has been employed. Oneof the limitations of this technique is the relative complexity of theset-up.

[0010] More recently, the amplification of pulses to peak powers higherthan 10 KW has been achieved in multi-mode fiber amplifiers. See U.S.Pat. No. 5,818,630, entitled Single-Mode Amplifiers and CompressorsBased on Multi-Mode Fibers, assigned to the assignee of the presentinvention, and hereby incorporated herein by reference. As describedtherein, the peak power limit inherent in single-mode optical fiberamplifiers is avoided by employing the increased area occupied by thefundamental mode within multi-mode fibers. This increased area permitsan increase in the energy storage potential of the optical fiberamplifier, allowing higher pulse energies before the onset ofundesirable nonlinearities and gain saturation. To accomplish this, thatapplication describes the advantages of concentration of the gain mediumin the center of the multi-mode fiber so that the fundamental mode ispreferentially amplified. This gain-confinement is utilized to stabilizethe fundamental mode in a fiber with a large cross section by gainguiding.

[0011] Additionally, that reference describes the writing of chirpedfiber Bragg gratings onto multi-mode fibers with reduced mode-couplingto increase the power limits for linear pulse compression of high-poweroptical pulses. In that system, double-clad multi-mode fiber amplifiersare pumped with relatively large-area high-power semiconductor lasers.Further, the fundamental mode in the multi-mode fibers is excited byemploying efficient mode-filters. By further using multi-mode fiberswith low mode-coupling, the propagation of the fundamental mode inmulti-mode amplifiers over lengths of several meters can be ensured,allowing the amplification of high-power optical pulses in dopedmulti-mode fiber amplifiers with core diameters of several tens ofmicrons, while still providing a diffraction limited output beam. Thatsystem additionally employed cladding pumping by broad area diode arraylasers to conveniently excite multi-mode fiber amplifiers.

[0012] Background Relating to Mode-locked Lasers

[0013] Both actively mode-locked lasers and passively mode-locked lasersare well known in the laser art. For example, compact mode-locked lasershave been formed as ultra-short pulse sources using single-moderare-earth-doped fibers. One particularly useful fiber pulse source isbased on Kerr-type passive mode-locking. Such pulse sources have beenassembled using widely available standard fiber components to providepulses at the bandwidth limit of rare-earth fiber lasers with GigaHertzrepetition rates.

[0014] Semiconductor saturable absorbers have recently foundapplications in the field of passively mode-locked, ultrashort pulselasers. These devices are attractive since they are compact,inexpensive, and can be tailored to a wide range of laser wavelengthsand pulsewidths. Quantum well and bulk semiconductor saturable absorbershave also been used to mode-lock color center lasers

[0015] A saturable absorber has an intensity-dependent loss l. Thesingle pass loss of a signal of intensity I through a saturable absorberof thickness d may be expressed as

l−1−exp(−αd)

[0016] in which α is the intensity dependent absorption coefficientgiven by:

α(I)−α₀/(1+I/I _(SAT))

[0017] Here α₀ is the small signal absorption coefficient, which dependsupon the material in question. I_(SAT) is the saturation intensity,which is inversely proportional to the lifetime (τ_(A)) of the absorbingspecies within the saturable absorber. Thus, saturable absorbers exhibitless loss at higher intensity.

[0018] Because the loss of a saturable absorber is intensity dependent,the pulse width of the laser pulses is shortened as they pass throughthe saturable absorber. How rapidly the pulse width of the laser pulsesis shortened is proportional to |dq₀/dI|, in which q₀ is the nonlinearloss:

q ₀ =l(I)−l(I=0)

[0019] l(I=0) is a constant (=1−exp(α₀d)) and is known as the insertionloss. As defined herein, the nonlinear loss q₀ of a saturable absorberdecreases (becomes more negative) with increasing intensity I. |dq₀/dI|stays essentially constant until I approaches I_(SAT), becomingessentially zero in the bleaching regime, i.e., when I>>I_(SAT).

[0020] For a saturable absorber to function satisfactorily as amode-locking element, it should have a lifetime (i.e., the lifetime ofthe upper state of the absorbing species), insertion loss l(I=0), andnonlinear loss q₀ appropriate to the laser. Ideally, the insertion lossshould be low to enhance the laser's efficiency, whereas the lifetimeand the nonlinear loss q₀ should permit self-starting and stable cwmode-locking. The saturable absorber's characteristics, as well as lasercavity parameters such as output coupling fraction, residual loss, andlifetime of the gain medium, all play a role in the evolution of a laserfrom startup to mode-locking.

[0021] As with single-mode fiber amplifiers, the peak-power of pulsesfrom mode-locked single-mode lasers has been limited by the small fibercore size that has been employed to ensure single-mode operation of thefiber. In addition, in mode-locked single-mode fiber lasers, theround-trip nonlinear phase delay also needs to be limited to around

to prevent the generation of pulses with a very large temporallyextended background, generally referred to as a pedestal. For a standardmode-locked single-mode erbium fiber laser operating at 1.55 μm with acore diameter of 10 μm and a round-trip cavity length of 2 m,corresponding to a pulse repetition rate of 50 MHz, the maximumoscillating peak power is thus about 1 KW.

[0022] The long-term operation of mode-locked single-mode fiber lasersis conveniently ensured by employing an environmentally stable cavity asdescribed in U.S. Pat. No. 5,689,519, entitled Environmentally StablePassively Mode-locked Fiber Laser Pulse Source, assigned to the assigneeof the present invention, and hereby incorporated herein by reference.The laser described in this reference minimizes environmentally inducedfluctuations in the polarization state at the output of the single-modefiber. In the described embodiments, this is accomplished by including apair of Faraday rotators at opposite ends of the laser cavity tocompensate for linear phase drifts between the polarization eigenmodesof the fiber.

[0023] Recently the reliability of high-power single-mode fiber laserspassively mode-locked by saturable absorbers has been greatly improvedby implementing non-linear power limiters by insertion of appropriatesemiconductor two-photon absorbers into the cavity, which minimizes thepeak power of the damaging Q-switched pulses often observed in thestart-up of mode-locking and in the presence of misalignments of thecavity. See U.S. patent application Ser. No. 09/149,369, filed on Sep.8, 1998, entitled Resonant Fabry-Perot Semiconductor Saturable Absorbersand Two-Photon Absorption Power Limiters, assigned to the assignee ofthe present invention, and hereby incorporated herein by reference.

[0024] To increase the pulse energy available from mode-lockedsingle-mode fiber lasers the oscillation of chirped pulses inside thelaser cavity has been employed. M. Hofer et al., Opt. Lett., vol. 17,page 807-809. As a consequence the pulses are temporally extended,giving rise to a significant peak power reduction inside the fiberlaser. However, the pulses can be temporally compressed down toapproximately the bandwidth limit outside the laser cavity. Due to theresulting high peak power, bulk-optic dispersive delay lines have to beused for pulse compression. For neodymium fiber lasers, pulse widths ofthe order of 100 fs can be obtained.

[0025] The pulse energy from mode-locked single-mode fiber lasers hasalso been increased by employing chirped fiber gratings. The chirpedfiber gratings have a large amount of negative dispersion, broadeningthe pulses inside the cavity dispersively, which therefore reduces theirpeak power and also leads to the oscillation of high-energy pulsesinside the single-mode fiber lasers.

[0026] See U.S. Pat. No. 5,450,427, entitled Technique for theGeneration of Optical Pulses in Mode-Locked Lasers by Dispersive Controlof the Oscillation Pulse Width, and U.S. Pat. No. 5,627,848, entitledApparatus for Producing Femtosecond and Picosecond Pulses from FiberLasers Cladding Pumped with Broad Area Diode Laser Arrays, both of whichare assigned to the assignee of the present invention and herebyincorporated herein by reference. In these systems, the generated pulsesare bandwidth-limited, though the typical oscillating pulse widths areof the order of a few ps.

[0027] However, though the dispersive broadening of the pulse widthoscillating inside a single-mode fiber laser cavity does increase theoscillating pulse energy compared to a ‘standard’ soliton fiber laser,it does not increase the oscillating peak power. The maximum peak powergenerated with these systems directly from the fiber laser is stilllimited to around I KW.

[0028] Another highly integratable method for increasing the peak powerof mode-locked lasers is based on using chirped periodically poledLiNb0₃ (chirped PPLN). Chirped PPLN permits simultaneous pulsecompression and frequency doubling of an optically chirped pulse. SeeU.S. patent application Ser. No. 08/845,410, filed on Apr. 25, 1997,entitled Use of Aperiodic Quasi-Phase-Matched Gratings in UltrashortPulse Sources, assigned to the assignee of the present application, andhereby incorporated herein by reference. However, for chirped PPLN toproduce pulse compression from around 3 ps to 300 fs and frequencydoubling with high conversion efficiencies, generally peak powers of theorder of several KW are required. Such high peak powers are typicallyoutside the range of mode-locked single-mode erbium fiber lasers.

[0029] Broad area diode laser arrays have been used for pumping ofmode-locked single-mode fiber lasers, where very compact cavity designswere possible. The pump light was injected through a V-groove from theside of double-clad fiber, a technique typically referred to asside-pumping. However, such oscillator designs have also suffered frompeak power limitations due to the single-mode structure of theoscillator fiber.

[0030] It has also been suggested that a near diffraction-limited outputbeam can be obtained from a multi-mode fiber laser when keeping thefiber length shorter than 15 mm and selectively providing a maximumamount of feedback for the fundamental mode of the optical fiber.“Efficient laser operation with nearly diffraction-limited output from adiode-pumped heavily Nd-doped multi-mode fiber”, Optics Letters, Vol.21, pp. 266-268 (1996) hereby incorporated herein by reference. In thistechnique, however, severe mode-coupling has been a problem, as theemployed multi-mode fibers typically support thousands of modes. Also,only an air-gap between the endface of the multi-mode fiber and a lasermirror has been suggested for mode-selection. Hence, only very poormodal discrimination has been obtained, resulting in poor beam quality.

[0031] While the operation of optical amplifiers, especially in thepresence of large seed signals, is not very sensitive to the presence ofspurious reflections, the stability of mode-locked lasers criticallydepends on the minimization of spurious reflections. Any strayreflections produce sub-cavities inside an oscillator and result ininjection signals for the cw operation of a laser cavity and thusprevent the onset of mode-locking. For solid-state Fabry-Perot cavitiesa suppression of intra-cavity reflections to a level <<1% (in intensity)is generally believed to be required to enable the onset ofmode-locking.

[0032] The intra-cavity reflections that are of concern in standardmode-locked lasers can be thought of as being conceptually equivalent tomode-coupling in multi-mode fibers. Any mode-coupling in multi-modefibers clearly also produces a sub-cavity with a cw injection signalproportional to the amount of mode-coupling. However, the suppression ofmode-coupling to a level of <<I% at any multi-mode fiber discontinuitiesis very difficult to achieve. Due to optical aberrations, evenwell-corrected optics typically allow the excitation of the fundamentalmode in multi-mode fibers only with maximum efficiency of about 95%.Therefore to date, it has been considered that mode-locking of amulti-mode fiber is impossible and no stable operation of a mode-lockedmulti-mode fiber laser has yet been demonstrated.

DESCRIPTION OF THE RELATED ART

[0033] Rare-earth-doped optical fibers have long been considered for useas sources of coherent light, as evidenced by U.S. Pat. No. 3,808,549 toMaurer (1974), since their light-guiding properties allow theconstruction of uniquely simple lasers. However, early work on fiberlasers did not attract considerable attention, because no methods ofgenerating diffraction-limited coherent light were known. Man currentapplications of lasers benefit greatly from the presence of diffract onlimited light.

[0034] Only when it became possible to manufacture single-mode (SM)rare-earth-doped fibers, as reported by Poole et al. in “Fabrication ofLow-Loss Optical Fibres Containing Rare-Ear Ions”, Optics Letters, Vol.22, pp. 737-738 (1985), did the rare-earth-doped optical fibertechnology become viable. In this technique, only the fundamental modeof the optical fiber is guided at the lasing wavelength, thus ensuringdiffraction-limited output.

[0035] Driven by the needs of optical fiber telecommunications for SMoptical fiber amplifiers, nearly all further developments for more thana decade in this area were concentrated on perfecting SM fiberamplifiers. In particular, the motivation for developing SM fiberamplifiers stemmed from the fact that SM fiber amplifiers generate theleast amount of noise and they are directly compatible with SM fiberoptic transmission lines. SM fiber amplifiers also have the highestoptical transmission bandwidths, since, due to the absence of anyhigher-order modes, modal dispersion is completely eliminated. Ingeneral, modal dispersion is the most detrimental effect limiting thetransmission bandwidth of multi-mode (MM) optical fibers, since thehigher-order modes, in general, have different propagation constants.

[0036] However, in the amplification of short-optical pulses, the use ofSM optical fibers is disadvantageous, cause the limited core area limitsthe saturation energy of the optical fiber and thus the obtainable pulseenergy. The saturation energy of a laser amplifier can be expressed as${E_{sat} = \frac{h\quad \upsilon \quad A}{\sigma}},$

[0037] where h is Planck's constant, υ is the optical frequency, a isthe stimulated emission cross section and A is the core area. Thehighest pulse energy generated from a SM optical fiber to date is about160 μJ (disclosed by Taverner et al. in Optics Letters, Vol. 22, pp.378-380 (1997), and was obtained from a SM erbium-doped fiber with acore diameter of 15 μm, which is about the largest core diameter that iscompatible with SM propagation at 1.55 μm. This result was obtained witha fiber numerical aperture of NA≈0.07. Any further increase in corediameter requires a further lowering of the NA of the fiber and resultsin an unacceptably high sensitivity to bend-losses.

[0038] As an alternative to SM amplifiers amplification in multi-mode(MM) optical fibers has been considered. See, for example,“Chirped-pulse amplification of ultrashort pulses with a multi-modeTm:ZBLAN fiber upconversion amplifier” by Yang et al., Optics Letters,Vol. 20, pp. 1044-1046 (1995). However, in general, amplificationexperiments in MM optical fibers have led to non-diffraction-limitedoutputs as well as unacceptable pulse broadening due to modaldispersion, since the launch conditions into the MM optical fiber andmode-coupling in the MM fiber were not controlled.

[0039] It was recently suggested by Griebner et al. in “Efficient laseroperation with nearly diffraction-limited output from a diode-pumpedheavily Nd-doped multi-mode fiber”, Optics Letters, Vol. 21, pp. 266-268(1996), that a near diffraction-limited output be can be obtained from aMM fiber laser when keeping the fiber length shorter than 15 mm andselectively providing a maximum amount of feedback for the fundamentalmode of the optical fiber. In this technique, however, severeode-coupling was a problem, as the employed MM fibers supported some10,000 modes. Also, only an air-gap between the endface of the MM fiberand a laser mirror was suggested for mode-selection. Hence, only verypoor modal discrimination was obtained, resulting in poor beam quality.

[0040] In U.S. Pat. No. 5,187,759 to DiGiovanni et al., it was suggestedthat amplified spontaneous emission (ASE) in a MM fiber can be reducedby selectively exciting any active ions lose to the center of the fibercore or by confining the active ions to the center of the fiber core.Since the overlap of the low-order modes in a MM optical fiber ishighest with the active ions close to the center of the fiber core, anyASE will then also be predominantly generated in low-order modes of theMM fiber. As a result, the total amount of ASE can be greatly reduced inMM fiber, since no ASE is generated in high-order modes. However,DiGiovanni described dopant confinement only with respect to ASEreduction. DiGiovanni did not suggest that, in the presence ofmode-scattering, dopant confinement can enhance the beam quality of thefundamental mode of the M fiber under SM excitation. Also, the system ofDiGiovanni did not take into account the fact that gain-guiding inducedby dopant confinement can in fact effectively guide a fundamental modein a MM fiber. This further reduces ASE in MM fibers as well as allowingfor SM operation.

[0041] In fact, the system of DiGiovanni et al. is not very practical,since it considers a MM signal source, which leads to anon-diffraction-limited output beam. Further, only a single cladding wasconsidered for the doped fiber, which is disadvantageous when trying tocouple high-power semi-conductor lasers into the optical fibers. Tocouple high-power semiconductor lasers into MM fibers, a double-cladstructure, as suggested in the above-mentioned patent to Maurer, can beof an advantage.

[0042] To the inventors' knowledge, gain-guiding has not previously beenemployed in optical fibers. On the other hand, gain-guiding is wellknown in conventional semiconductor and solid-state lasers. See, forexample, “Alexandrite-laser-pumped Cr³⁺:Li rA1F₆” by Harter et al.,Optics Letters, Vol. 17, pp. 1512-1514 (1992). Indeed, in SM fibers,gain-guiding is irrelevant due to the strong confinement of thefundamental mode by the wave-guide structure. However, in MM opticalfibers., the confinement of the fundamental mode by the waveguidestructure becomes comparatively weaker, allowing for gain-guiding to setin. As the core size in a MM fiber becomes larger, light propagation inthe fiber structure tends to approximate free-space propagation. Thus,gain-guiding can be expected eventually to be significant, providedmode-coupling can be mad sufficiently small. In addition to providinghigh pulse energies, MM optical fiber amplifiers can also be used toamplify very high peak power pulses due to their increased fiber crosssection compared to SM fiber amplifiers. MM undoped fibers and MMamplifier fibers can also be used for pulse compression as recentlydisclosed by Fermann et al. in U.S. application Ser. No. 08/789,995(filed Jan. 28, 1997). However, this work was limited to the use of MMfibers as soliton Raman compressors in conjunction with a nonlinearspectral filtering action to clean-up the spectral profile, which maylimit the overall efficiency of the system.

[0043] Compared to pulse compression in SM fibers, such as thatdisclosed in U.S. Pat. No. 4,913,520 to Kafka et al., higher-pulseenergies can be obtained in MM fibers due to the increased mode-size ofthe fiber. In particular, V-values higher than 2.5 and relatively highindex differences between core and cladding (i.e. a Δn>0.3%) can beeffectively employed. In “Generation of high-energy 10-fs pulses by anew pulse compression technique”, Conference on Lasers andElectro-Optics, CLEO 91, paper DTuR5, Optical Society of AmericaTechnical Digest Series, #9, pp. 189-190 (1996), M. Nisoli et al.suggested the use of hollow-core fibers for pulse-compression, ashollow-core fibers allow an increase in the mode size of the fundamentalmode. However, hollow-core fibers have an intrinsic transmission loss,they need to be filled with gas, and they need to be kept straight inorder to minimize the transmission losses, which makes them highlyimpractical.

[0044] As an alternative to obtaining high-power pulses, chirped pulseamplification with chirped fiber Bragg gratings may be employed, asdisclosed in U.S. Pat. No. 5,499,134 to Galvanauskas et al. (1996). Oneof the limitations of this technique is that, in the compressiongrating, a SM fiber with a limited core area is employed. Higher pulseenergies could be obtained by employing chirped fiber Bragg gratings inMM fibers with reduced mode-coupling for pulse compression. Indeed,unchirped fiber Bragg gratings were recently demonstrated in double-modefibers by Strasser et al. in “Reflective-mode conversion with UV-inducedphase gratings in two-mode fiber”, Optical Society of America Conferenceon Optical Fiber Communication, OFC97, pp. 348-349, (1997). However,these gratings were blazed to allow their use as mode-converters, i.e.,to couple the fundamental mode to a higher-order mode. The use of Bragggratings in pulse-compression calls for an unblazed grating to minimizethe excitation of any higher-order modes in reflection.

[0045] It has long been known that a SM signal can be coupled into a MMfiber structure and preserved for propagation lengths of 100 s ofmeters. See; for example, “Pulse Dispersion for Single-Mode Operation ofMulti-mode Cladded Optical Fibres”, Gambling et al., Electron. Lett.,Vol. 10, pp. 148-149, (1974) and “Mode conversion coefficients inoptical fibers”, Gambling et al., Applied Optics, Vol. 14, pp.1538-1542, (1975). However, Gambling et al. found low levels ofmode-coupling only in liquid-core fibers. On the other hand,mode-coupling in MM solid-core fibers was found to be severe, allowingfor the propagation of a fundamental mode only in mm lengths of fiber.Indeed, as with the work by Griebner et al., Gambling et al. used MMsolid-core optical fibers that supported around 10,000 or more modes.

[0046] In related work, Gloge disclosed in “Optical Power Flow inMulti-mode Fibers”, The Bell System Technical Journal, Vol. 51, pp.1767-1783, (1972), the use of MM fibers that supported only 700 modes,where mode-coupling was sufficiently reduced to allow SM propagationover fiber lengths of 10 cm.

[0047] However, it was not shown by Gloge that mode-coupling can bereduced by operating MM fibers at long wavelengths (1.55 μm) and byreducing the total number of modes to less than 700. Also, in this work,the use of MM fibers as amplifiers and the use of the nonlinearproperties of MM fibers was not considered.

[0048] The inventors are not aware of any prior art using MM fibers toamplify SM signals where the output remains primarily in the fundamentalmode, the primary reason being that amplification in MM fibers istypically not suitable for long-distance signal propagation as employedin the optical telecommunication area. The inventors arc also not awareof any prior art related to pulse compression in multi-mode fibers,where the output remains in the fundamental mode.

[0049] All of the above-mentioned articles, patents and patentapplications are hereby incorporated herein by reference.

SUMMARY OF THE INVENTION

[0050] This invention overcomes the foregoing difficulties associatedwith peak power limitations in mode-locked lasers, and provides amode-locked multi-mode fiber laser.

[0051] This laser utilizes cavity designs which allow the stablegeneration of high peak power pulses from mode-locked multi-mode fiberlasers, greatly extending the peak power limits of conventionalmode-locked single-mode fiber lasers. Mode-locking may be induced byinsertion of a saturable absorber into the cavity and by inserting oneor more mode-filters to ensure the oscillation of the fundamental modein the multi-mode fiber. The probability of damage of the absorber maybe minimized by the insertion of an additional semiconductor opticalpower limiter into the cavity. The shortest pulses may also be generatedby taking advantage of nonlinear polarization evolution inside thefiber. The long-term stability of the cavity configuration is ensured byemploying an environmentally stable cavity. Pump light from a broad-areadiode laser may be delivered into the multi-mode fiber by employing acladding-pumping technique.

[0052] With this invention, a mode-locked fiber laser may be constructedto obtain, for example, 360 fsec near-bandwidth-limited pulses with anaverage power of 300 mW at a repetition rate of 66.7 MHz. The peak powerof these exemplary pulses is estimated to be about 6 KW.

[0053] It is an object of the present invention to increase the energystorage potential in an optical fiber amplifier and to produce peakpowers and pulse energies which are higher than those achievable insingle-mode (SM) fibers before the onset of undesirable nonlinearitiesand gain saturation.

[0054] Another object of the present invention is to achieveamplification of the fundamental mode within a multi-mode (MM) fiberwhile reducing amplified spontaneous emission (ASE).

[0055] A further object of the present invention is to employgain-guiding within a MM fiber to improve the stability of thefundamental mode.

[0056] Yet another object of the present invention is to compress highpeak power pulses into the range of a few psec to a fsec whilepreserving a near diffraction-limited output.

[0057] To achieve the above objects, the present invention employs amulti-mode (MM) optical fiber in an optical amplification system.According to the present invention, MM optical fibers, i.e., fibers witha V-value greater than approximately 2.5, provide an output in thefundamental mode. This allows the generation of much higher peak powersand pulse energies compared to SM fibers before the onset of undesirablenonlinearities and gain saturation. The increased fiber cross sectionequally greatly increases the energy storage potential in an opticalfiber amplifier. The amplification system of the present invention isuseful in applications requiring ultrafast and high-power pulse sources.

[0058] According to one aspect of the present invention, the gain mediumis in the center of the MM fiber so that the fundamental mode ispreferentially amplified and spontaneous emission is reduced. Further,gain-confinement is used to stabilize the fundamental mode in a fiberwith a large cross section by gain guiding.

[0059] According to one embodiment of the present invention, theexploitation of self-phase modulation and other nonlinearities in(rare-earth) doped or undoped MM fibers allows the compression of highpeak power pulses into the range of a few fsec while a neardiffraction-limited output is pre-served.

[0060] According to another embodiment of the present invention, bywriting chirped fiber Bragg gratings into MM optical fibers with reducedmode-coupling, the power limits for linear pulse compression ofhigh-power optical pulses are greatly increased. Further, by employingdouble-clad MM fiber amplifiers, pumping with relatively large-areahigh-power semiconductor lasers is made possible.

[0061] According to yet another embodiment of the present invention, theincorporation of efficient mode-filters enables cw lasing in a neardiffraction-limited single mode from (rare-earth) doped MM opticalfibers.

[0062] According to yet another embodiment of the present invention, MMoptical fibers allow the construction of fiber optic regenerativeamplifiers and high-power Q-switched lasers. Further, MM optical fibersallow the design of cladding-pumped fiber lasers using dopants withrelatively weak absorption cross sections.

[0063] These and other objects and features of the present inventionwill be apparent from the following detailed description of thepreferred embodiments and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0064] The following description of the preferred embodiments of theinvention references the appended drawings, in which like elements bearidentical reference numbers throughout.

[0065]FIG. 1 is a schematic illustration showing the construction of apreferred embodiment of the present invention which utilizes end-pumpingfor injecting pump light into the multi-mode fiber.

[0066]FIG. 2 is a graph showing the typical autocorrelation of pulsesgenerated by the invention of FIG. 1.

[0067]FIG. 3 is a graph showing the typical pulse spectrum generated bythe invention of FIG. 1.

[0068]FIG. 4 is a schematic illustration showing the construction of analternate preferred embodiment utilizing a side-pumping mechanism forinjecting pump light into the multi-mode fiber.

[0069]FIG. 5 is a schematic illustration of an alternative embodimentwhich uses a length of positive dispersion fiber to introduce chirpedpulses into the cavity.

[0070]FIG. 6 is a schematic illustration of an alternative embodimentwhich uses chirped fiber gratings with negative dispersion in the lasercavity to produce high-energy, near bandwidth-limited pulses.

[0071]FIGS. 7a and 7 b illustrate polarization-maintaining multi-modefiber cross sections which may be used to construct environmentallystable cavities in the absence of Faraday rotators.

[0072]FIG. 8 is a schematic illustration of an alternative embodimentwhich utilizes one of the fibers illustrated in FIGS. 7a and 7 b.

[0073]FIGS. 9a, 9 b and 9 c illustrate the manner is which thefundamental mode of the multi-mode fibers of the present invention maybe matched to the mode of a singe mode fiber. These include a bulk opticimaging system, as shown in FIG. 9a, a multi-mode to single-mode splice,as shown in FIG. 9b, and a tapered section of multi-mode fiber, asillustrated in FIG. 9c.

[0074]FIG. 10 is a schematic illustration of an alternative embodimentin which a fiber grating is used to predominantly reflect thefundamental mode of a multi-mode fiber.

[0075]FIG. 11 is a schematic illustration of an alternative embodimentin which active or active-passive mode-locking is used to mode-lock themulti-mode laser.

[0076]FIG. 12 is a diagrammatic view of a multi-mode fiber amplifiersystem according to the first embodiment of the present invention.

[0077]FIG. 13 is a graph showing the coupling efficiency of a multi-modeamplifier fiber into a mode-filter fiber as a function of bend-radius ofthe multi-mode amplifier fiber.

[0078]FIG. 14 is a graph showing the autocorrelation of the amplifiedpulses from a multi-mode amplifier fiber measured under optimummode-match conditions.

[0079]FIG. 15 is a graph showing the autocorrelation of the amplifiedpulses from a multi-mode amplifier fiber measured under non-optimummode-match conditions.

[0080]FIG. 16 is a block diagram of a multi-mode fiber amplifier systemaccording to the second embodiment of the present invention.

[0081]FIG. 17 is a block diagram of a multi-mode fiber amplifier systemaccording to the third embodiment of the present invention, wherein apulse compressor is disposed at an output of the multi-mode fiber.

[0082]FIG. 18 is a diagrammatic view of a multi-mode fiber amplifiersystem according to a fourth embodiment of the present invention.

[0083]FIG. 19 is a conceptual drawing of a fiber cross section employinga doped multi-mode fiber core and an undoped fiber cladding according toa fifth embodiment of the present invention.

[0084]FIG. 20 is a diagrammatic view of a multi-mode fiber amplifiersystem according to a sixth embodiment of the present invention, whereina fiber regenerative amplifier is constructed from a multi-mode fiberamplifier.

[0085]FIG. 21 is a diagrammatic view of a multi-mode fiber amplifiersystem according to a seventh embodiment of the present invention,wherein a MM Q-switched fiber laser source is constructed.

[0086]FIG. 22 is a block diagram of a multi-mode fiber amplifier systemaccording to the eighth embodiment of the present invention, wherein apreamplifier is inserted before the multi-mode fiber.

[0087]FIG. 23 is a block diagram of a multi-mode fiber amplifier systemaccording to the ninth embodiment of the present invention, wherein afrequency converter is disposed at an output of the multi-mode fiber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0088]FIG. 1A illustrates the mode-locked laser cavity 11 of thisinvention which uses a length of multi-mode amplifying fiber 13 withinthe cavity to produce ultra-short, high-power optical pulses. As usedherein, “ultra-short” means a pulse width below 100 ps. The fiber 13, inthe example shown, is a 1.0 m length of non-birefringent Yb³⁺/Er³⁺-dopedmulti-mode fiber. Typically, a fiber is considered multi-mode when theV-value exceeds 2.41, i.e., when modes in addition to the fundamentalmode can propagate in the optical fiber. This fiber is coiled onto adrum with a diameter of 5 cm, though bend diameters as small as 1.5 cm,or even smaller, may be used without inhibiting mode-locking. Due to theEr³⁺ doping, the fiber core in this example has an absorption ofapproximately 40 dB/m at a wavelength of 1.53 μm. The Yb³⁺ co-dopingproduces an average absorption of 4.3 dB/m inside the cladding at awavelength of 980 nm. The fiber 13 has a numerical aperture of 0.20 anda core diameter of 16 μm. The outside diameter of the cladding of thefiber 13 is 200 μm. The fiber 13 is coated with a low-index polymerproducing a numerical aperture of 0.40 for the cladding. A 10 cm lengthof single-mode Corning Leaf fiber 15 is thermally tapered to produce acore diameter of approximately 14 μm to ensure an optimum operation as amode filter, and this length is fusion spliced onto a first end 17 ofthe multi-mode fiber 13.

[0089] In this exemplary embodiment, the cavity 11 is formed between afirst mirror 19 and a second mirror 21. It will be recognized that othercavity configurations for recirculating pulses are well known, and maybe used. In this example, the mirrors 19, 21 define an optical axis 23along which the cavity elements are aligned.

[0090] The cavity 11 further includes a pair of Faraday rotators 25, 27to compensate for linear phase drifts between the polarizationeigenmodes of the fiber, thereby assuring that the cavity remainsenvironmentally stable. As referenced herein, the phrase“environmentally stable” refers to a pulse source which is substantiallyimmune to a loss of pulse generation due to environmental influencessuch as temperature drifts and which is, at most, only slightlysensitive to pressure variations. The use of Faraday Rotators forassuring environmental stability is explained in more detail in U.S.Pat. No. 5,689,519 which has been incorporated by reference herein.

[0091] A polarization beam-splitter 29 on the axis 23 of the cavity 11ensures single-polarization operation of the cavity 11, and provides theoutput 30 from the cavity. A half-wave plate 31 and a quarter-wave plate33 are used to introduce linear phase delays within the cavity,providing polarization control to permit optimization of polarizationevolution within the cavity 11 for mode-locking.

[0092] To induce mode-locking, the cavity 11 is formed as a Fabry-Perotcavity by including a saturable absorber 35 at the end of the cavityproximate the mirror 19. The saturable absorber 35 is preferably grownas a 0.75 μm thick layer of InGaAsP on one surface of a substrate. Theband-edge of the InGaAsP saturable absorber 39 is preferably chosen tobe 1.56 μm, the carrier life-time is typically 5 ps and the saturationenergy density is 100 MW/cm².

[0093] In this example, the substrate supporting the saturable absorber35 comprises high-quality anti-reflection-coated InP 37, with theanti-reflection-coated surface 39 facing the open end of the cavity 11.The InP substrate is transparent to single-photon absorption of thesignal light at 1.56 μm, however two photon absorption occurs. Thistwo-photon absorber 39 is used as a nonlinear power limiter to protectthe saturable absorber 35.

[0094] The mirror 19 in this exemplary embodiment is formed bydepositing a gold-film onto the surface of the InGaAsP saturableabsorber 35 opposite the two photon absorber 39. The combined structureof the saturable absorber 35, two photon absorber 37 and mirror 19provides a reflectivity of 50% at 1.56 μm. The surface of the gold-filmmirror 19 opposite the saturable absorber 35 is attached to a sapphirewindow 41 for heat-sinking the combined absorber/mirror assembly.

[0095] The laser beam from the fiber 15 is collimated by a lens 43 andrefocused, after rotation by the Faraday rotator 25, by a lens 45 ontothe anti-reflection-coated surface 39 of the two-photon absorber 37. Thespot size of the laser beam on the saturable absorber 35 may be adjustedby varying the position of the lens 45 or by using lenses with differentfocal lengths. Other focusing lenses 47 and 49 in the cavity 11 aid inbetter imaging the laser signal onto the multi-mode fiber 13.

[0096] Light from a Pump light source 51, such as a laser source, with awavelength near 980 nm and output power of 5 W, is directed through afiber bundle 57 with an outside diameter of 375 μm. This pump light isinjected into the end 53 of the multi-mode fiber 13 opposite thesingle-mode fiber 17. The pump light is coupled into the cavity 11 via apump signal injector 55, such as a dichroic beam-splitter for 980/1550nm. Lenses 47 and 48 are optimized for coupling of the pump power fromthe fiber bundle 57 into the cladding of the multi-mode fiber.

[0097] The M²-value of the beam at the output 30 of this exemplaryembodiment is typically approximately 1.2. Assuming the deterioration ofthe M²-value is mainly due to imperfect splicing between the multi-modefiber 13 and the single-mode mode-filter fiber 15, it can be estimatedthat the single-mode mode-filter fiber 15 excited the fundamental modeof the multi-mode fiber 13 with an efficiency of approximately 90%.

[0098] Mode-locking may be obtained by optimizing the focussing of thelaser beam on the saturable absorber 35 and by optimizing theorientation of the intra-cavity waveplates 31, 33 to permit some degreeof nonlinear polarization evolution. However, the mode-locked operationof a multi-mode fiber laser system without nonlinear polarizationevolution can also be accomplished by minimizing the amount ofmode-mixing in the multi-mode fiber 13 and by an optimization of thesaturable absorber 35.

[0099] The pulses which are generated by the exemplary embodiment ofFIG. 1 will have a repetition rate of 66.7 MHz, with an average outputpower of 300 mW at a wavelength of 1.535 μm, giving a pulse energy of4.5 nJ. A typical autocorrelation of the pulses is shown in FIG. 2. Atypical FWHM pulse width of 360 fsec (assuming a sech² pulse shape) isgenerated. The corresponding pulse spectrum is shown in FIG. 3. Theautocorrelation width is within a factor of 1.5 of the bandwidth limitas calculated from the pulse spectrum, which indicates the relativelyhigh quality of the pulses.

[0100] Due to the multi-mode structure of the oscillator, the pulsespectrum is strongly modulated and therefore the autocorrelationdisplays a significant amount of energy in a pulse pedestal. It can beestimated that the amount of energy in the pedestal is about 50%, whichin turn gives a pulse peak power of 6 KW, about 6 times larger than whatis typically obtained with single-mode fibers at a similar pulserepetition rate.

[0101] Neglecting the amount of self-phase modulation in one passthrough the multi-mode fiber 13 and any self-phase modulation in themode-filter 15, and assuming a linear increase of pulse power in themulti-mode fiber 13 in the second pass, and assuming an effectivefundamental mode area in the multi-mode fiber 13 of 133 μm², thenonlinear phase delay in the multi-mode oscillator is calculated fromthe first equation above as Φ_(nl)=1.45π, which is close to the expectedmaximum typical nonlinear delay of passively mode-locked lasers.

[0102] The modulation on the obtained pulse spectrum as well as theamount of generated pedestal is dependent on the alignment of the mirror21. Generally, optimized mode-matching of the optical beam back into thefundamental mode of the multi-mode fiber leads to the best laserstability and a reduction in the amount of pedestal and pulse spectrummodulation. For this reason, optimized pulse quality can be obtained byimproving the splice between the single-mode filter fiber 15 and themulti-mode fiber 13. From simple overlap integrals it can be calculatedthat an optimum tapered section of Corning SMF-28 fiber 15 will lead toan excitation of the fundamental mode in the multi-mode fiber 13 with anefficiency of 99%. Thus any signal in higher-order modes can be reducedto about 1% in an optimized system.

[0103] An alternate embodiment of the invention is illustrated in FIG.4. As indicated by the identical elements and reference numbers, most ofthe cavity arrangement in this figure is identical to that shown inFIG. 1. This embodiment provides a highly integrated cavity 59 byemploying a side-pumping mechanism for injecting pump light into themulti-mode fiber 13. A pair of fiber couplers 61, 63, as are well knownin the art, inject light from a respective pair of fiber bundles 65 and67 into the cladding of the multi-mode fiber 13. The fiber bundles aresimilar to bundle 57 shown in FIG. 1, and convey light from a pair ofpump sources 69 and 71, respectively. Alternatively, the fiber bundles65, 67 and couplers 61, 63 may be replaced with V-groove light injectioninto the multi-mode fiber cladding in a manner well known in the art. Asaturable absorber 73 may comprise the elements 35, 37, 39 and 41 shownin FIG. 1, or may be of any other well known design, so long as itprovides a high damage threshold.

[0104] In another alternate embodiment of the invention, Illustrated inFIG. 5, the laser cavity 75 includes a positive dispersion element. AsWith FIG. 4, like reference numbers in FIG. 5 identify elementsdescribed in detail with reference to FIG. 1. In this embodiment, asection of single-mode positive dispersion fiber 77 is mounted betweenthe second mirror 21 and the lens 49. In a similar manner, a section ofpositive dispersion fiber could be spliced onto the end 53 of themulti-mode fiber 13, or the end of the single-mode mode-filter 15 facingthe lens 43. Positive dispersion fibers typically have a small corearea, and may limit the obtainable pulse energy from a laser. Theembodiment shown in FIG. 5 serves to reduce the peak power injected intothe positive dispersion fiber 77, and thus maximize the pulse energyoutput.. This is accomplished by extracting, at the polarization beamsplitter 29, as much as 90-99% of the light energy.

[0105] In the embodiment of FIG. 5, the total dispersion inside thecavity may be adjusted to be zero to generate high-power pulses With alarger bandwidth. Alternatively, by adjusting the total cavitydispersion to be positive, chirped pulses with significantly increasedpulse energies may be generated by the laser.

[0106] The use of two single-mode mode-filter fibers 15, 77 is alsobeneficial in simplifying the alignment of the laser. Typically, tominimize modal speckle, broad bandwidth optical signals need to be usedfor aligning the mode-filter fibers with the multi-mode fiber. The useof two mode-filter fibers 15, 77 allows the use of amplified spontaneousemission signals generated directly in the multi-mode fiber for aniterative alignment of both mode-filters 15, 77.

[0107] The chirped pulses generated in the cavity 75 with overallpositive dispersion may be compressed down to approximately thebandwidth limit at the frequency doubled wavelength by employing chirpedperiodically poled LiNbO₃ 79 for sum-frequency generation, in a mannerwell known in the art. The chirped periodically poled LiNbO₃ 79 receivesthe cavity output from the polarization beam splitter 29 through anoptical isolator 81. In this case, due to the high power capabilities ofmulti-mode fiber oscillators, higher frequency-doubling conversionefficiencies occur compared to those experienced with single-mode fiberoscillators. Alternatively, bulk-optics dispersion compensating elementsmay be used in place of the chirped periodically poled LiNbO₃ 79 forcompressing the chirped pulses down to the bandwidth limit.

[0108] Generally, any nonlinear optical mixing technique such asfrequency doubling, Raman generation, four-wave mixing, etc. may be usedin place of the chirped periodically poled LiNbO₃ 79 to frequencyconvert the output of the multi-mode oscillator fiber 13 to a differentwavelength. Moreover, the conversion efficiency of these nonlinearoptical mixing processes is generally proportional to the lightintensity or light intensity squared. Thus, the small residual pedestalpresent in a multi-mode oscillator would be converted with greatlyreduced efficiency compared to the central main pulse and hence muchhigher quality pulses may be obtained.

[0109] As shown in the alternate embodiment of FIG. 6, very high-energyoptical pulses may also be obtained by inserting a chirped fiber gratingsuch as a Bragg grating 83, with negative dispersion, into the cavity85. Such a system typically produces ps length, high-energy,approximately bandwidth-limited pulses. Due to the multi-mode fiberused, much greater peak powers compared to single-mode fiber oscillatorsare generated. Here the fiber grating 83 is inserted after thepolarization beam splitter 29 to obtain an environmentally-stable cavityeven in the presence of nonpolarization maintaining multi-mode fiber 13.

[0110] In each of the embodiments of this invention, it is advantageousto minimize saturation of the multi-mode fiber amplifier 13 by amplifiedspontaneous emission generated in higher-order modes. This may beaccomplished by confining the rare-earth doping centrally within afraction of the core diameter.

[0111] Polarization-maintaining multi-mode optical fiber may beconstructed by using an elliptical fiber core or by attachingstress-producing regions to the multi-mode fiber cladding. Examples ofsuch fiber cross-sections are shown in FIGS. 7a and 7 b, respectively.Polarization-maintaining multi-mode fiber allows the construction ofenvironmentally stable cavities in the absence of Faraday rotators. Anexample of such a design is shown in FIG. 8 in this case, the output ofthe cavity 87 is provided by using a partially-reflecting mirror 89 atone end of the cavity 87, in a manner well known in this art.

[0112] To ensure optimum matching of the fundamental mode of themulti-mode fiber 13 to the mode of the single-mode mode-filter fiber 15in each of the embodiments of this invention, either a bulk opticimaging system, a splice between the multi-mode fiber 13 and thesingle-mode fiber 15, or a tapered section of the multi-mode fiber 13may be used. For example, the multi-mode fiber 13, either in the formshown in one for FIG. 7a and FIG. 7b or in a non-polarizationmaintaining form may be tapered to an outside diameter of 70 μm. Thisproduces an inside core diameter of 5.6 μm and ensures single modeoperation of the multi-mode fiber at the tapered end. By furtheremploying an adiabatic taper, the single-mode of the multi-mode fibermay be excited with nearly 100% efficiency. A graphic representation ofthe three discussed methods for excitation of the fundamental mode in anmulti-mode fiber 13 with a single-mode fiber mode-filter 15 is shown inFIGS. 9a, 9 b and 9 c, respectively. The implementation in a cavitydesign is not shown separately, but the splice between the single-modefiber 15 and the multi-mode fiber 15 shown in any of the disclosedembodiments may be constructed with any of the three alternatives shownin these figures.

[0113]FIG. 10 shows an additional embodiment of the invention. Here,instead of single-mode mode-filter fibers 15 as used in the previousembodiments, fiber gratings such as a Bragg grating directly writteninto the multi-mode fiber 13 is used to predominantly reflect thefundamental mode of the multi-mode fiber 13. Light from the pump 51 isinjected through the fiber grating 97 to facilitate a particularlysimple cavity design 99. Both chirped fiber gratings 97 as well asunchirped gratings can be implemented. Narrow bandwidth (chirped orunchirped) gratings favor the oscillation of pulses with a bandwidthsmaller than the grating bandwidth.

[0114] Finally, instead of passive mode-locking, active mode-locking oractive-passive mode-locking techniques may be used to mode-lockmulti-mode fibers. For example, an active-passive mode-locked systemcould comprise an optical frequency or amplitude modulator (as theactive mode-locking mechanism) in conjunction with nonlinearpolarization evolution (as the passive mode-locking mechanism) toproduce short optical pulses at a fixed repetition rate without asaturable absorber. A diagram of a mode-locked multi-mode fiber 13 witha optical mode-locking mechanism 101 is shown in FIG. 11. Also shown isan optical filter 103, which can be used to enhance the performance ofthe mode-locked laser 105.

[0115] Generally, the cavity designs described herein are exemplary ofthe preferred embodiments of this invention. Other variations areobvious from the previous discussions. In particular, opticalmodulators, optical filters, saturable absorbers and a polarizationcontrol elements are conveniently inserted at either cavity end.Equally, output coupling can be extracted at an optical mirror, apolarization beam splitter or also from an optical fiber couplerattached to the single-mode fiber filter 15. The pump power may also becoupled into the multi-mode fiber 13 from either end of the multi-modefiber 13 or through the side of the multi-mode fiber 13 in any of thecavity configurations discussed. Equally, all the discussed cavities maybe operated with any amount of dispersion. Chirped and unchirpedgratings may be implemented at either cavity end to act as opticalfilters and also to modify the dispersion characteristics of the cavity.

[0116]FIG. 12 illustrates an amplifier system according to a firstembodiment of the present invention. In the example shown in FIG. 12, afemtosecond single-mode (SM) fiber oscillator 1010, such as an erbiumfiber oscillator, is coupled into a multi-mode (MM) fiber amplifier1012, such as an erbium/ytterbium fiber amplifier. Other examples ofsuitable MM fiber amplifiers include those doped with Er, Yb, Nd, Tm, Pror Ho ions. Oscillators suitable for use in this system are described inthe above-mentioned U.S. patent application Ser. No. 08/789,995 toFermann et al.

[0117] A two-lens telescope 14 (L1 and L2) is used to match the modefrom the oscillator 1010 to the fundamental mode of the MM amplifier1012. In addition, the output of the pumped MM fiber 1012 is imaged intoa second SM fiber (mode-filter (MF) fiber 1016 in FIG. 12) using lensesL3 and L4. Lenses L3 and L5 and beamsplitter 1018 are used to couple thepump light from pump source 1020 into the amplifier fiber, as describedbelow.

[0118] In one example of the system arranged according to FIG. 12, theoscillator 1010 delivers 300 fsec near bandwidth-limited pulses at arepetition rate of 100 MHz at a wavelength of 1.56 μm with a power levelof 14 mW.

[0119] The amplifier fiber 12 can be, for example, a double-clad MMerbium/ytterbium amplifier with a core diameter of ≈28 μm and a corenumerical aperture of NA=0.19. The inner cladding in this example has adiameter of ≈220 μm and a numerical aperture of NA=0.24. The core islocated in the center of the inner cladding. The length of the amplifieris 1.10 m.

[0120] To increase the number of propagating modes in the MM amplifier1012 and for testing purposes, shorter wavelengths such as 780 and 633nm were also used. In this, a femto-second laser source operating at 780nm and a cw laser source at 633 nm can be launched into the MM amplifierfiber 1012. The MF fiber 1016 can then be replaced with a fiber with acore diameter of 4 μm to ensure SM operation at these two wavelengths.

[0121] The approximate number of modes in the MM amplifier is calculatedfrom its V-value. $\begin{matrix}{{v = {\frac{2\quad \pi \quad a}{\lambda}{NA}}},{{{number}\quad {of}\quad {modes}} = {\frac{1}{2}v^{2}}}} & (1)\end{matrix}$

[0122] where a is the core radius and λ is the signal wavelength. TheV-value at 1.55 μm is thus V≈10.8, and the number of modes is hencecalculated as ≈58 for the above example. Typically, a fiber isconsidered MM when the V-value exceeds 2.41, i.e., when modes inaddition to the fundamental mode can propagate in the optical fiber.

[0123] For equal excitation of N modes of a MM fiber supporting N modesthe maximum coupling efficiency into a SM fiber is given approximatelyby

η≈(θ₀/θ_(max))²≈1/N,  (2)

[0124] where θ₀≈λ/4a is the divergence half-angle of the fundamentalmode of the MM fiber. θ_(max) is the maximum divergence half-angle ofthe outer-most modes of the MM fiber. It is assumed that the output fromthe MM fiber is linearly polarized which is an appropriate assumptionfor the excitation of the lowest order modes in the fiber. Under SMexcitation of the MM fiber and in the absence of mode-coupling,θ_(max)(Z)−θ₀ independent of fiber length. However, in the presence ofmode-coupling θ_(max) will increase, and, as a result, the possiblecoupling efficiency from the output of the MM fiber into a SM fiber willdecrease as η(z)=(θ₀/θ_(max)(z))². Using the above-mentioned work byGloge, η(z) can be written as: $\begin{matrix}{{\eta (z)} = \frac{\theta_{0}^{1}}{{4\quad D\quad z} + \theta_{0}^{2}}} & (3)\end{matrix}$

[0125] where D is the mode-coupling coefficient as defined by Gloge.Thus, a measurement of η(z) gives the mode-coupling coefficient D.Equally, from equation (2), a measurement of η gives the approximatenumber of excited modes of a MM fiber. It is instructive to relate N tothe M²-value that is typically used to characterize the quality ofnear-diffraction-limited optical beams. It may be shown that N≈{squareroot}{square root over (M²)}. According to the present invention, a lowlevel of mode-coupling is desirable, so that the amplified beam providedat the output of the MM fiber amplifier 1012 is substantially in thefundamental mode. Accordingly, an M²-value less than 10 is desirable,with an M²-value less than 4 being preferable, and an M²-value less than2 being more preferable. Further, the number of modes is preferably inthe range of 3 to 3000 and more preferably in the range of 3 to 1000.

[0126] Mode-coupling was measured in a 1.1 m length of unpumpedamplifier fiber for the above-described erbium/ytterbium fiber (fiber1), and three commercially available MM-fibers (fiber 2, 3 and 4). Thefiber parameters and the mode-coupling coefficient D (in m⁻¹) of thesefibers are shown in Table 1. Fibers 1, 3 and 4 are made by the MCVDprocess; fiber 2 is made by a rod-in-tube technique. TABLE 1 fiber 1fiber 2 fiber 3 fiber 4 NA 0.19 0.36 0.13 0.13 core diameter (μm) 28 5050 50 cladding diameter 200 125 125 250 (μm) number of modules at 58 66587 87 1.55 μm number of modes at 223 0.79 μm number of modes at 330 0.63μm D(m⁻¹) at 1.55 μm <2 × 10⁻⁶  8 × 10⁻⁴ 8 × 10⁻⁵ 7 × 10⁻⁶ D(m⁻¹) at0.79 μm 4 × 10⁻⁶ D(m⁻¹) at 0.63 μm 2 × 10⁻⁵ L_(b)(mm) at 1.55 μm 1.9 5.35.7 5.7 L_(b)(mm) at 0.79 μm 3.3 L_(b)(mm) at 0.63 μm 4.1 M²(1 m) at1.55 μm 1.0 200 5.4 1.25 M²(1 m) at 0.79 μm 1.2 M²(1 m) at 0.63 μm 2.6

[0127] The coupling coefficients allow, in turn, the calculation of theexpected M² value. In this example, the calculated M²-values wereproduced after propagation through 1 m of MM fiber 1012. For fiber 1, agood agreement between the calculated and separately measured M²-valueswas obtained.

[0128] The beat length L_(b) between the fundamental LP₀₁ and the nexthigher-order LP₁₁ mode is also given in Table 1. The beat length L_(b)is defined as the length it takes for the two modes to accumulate adifferential phase-shift of 2π along the propagation direction. Assuminga constant scattering power spectrum, for a fixed wavelength, D can beshown to be proportional to L_(b) ⁴.

[0129] See: D. Marcuse, “The Theory of Dielectric Optical Waveguides”,p. 238, Academic Press (1974); Gloge. The longer the beat length, thecloser the modes are to being phase-matched and the more power willcouple as a function of length. Since, as disclosed by Gloge,mode-coupling is expected to be largest between adjacent modes, it isdesirable to use LP₀₁/LP₁₁ beat lengths as short as possible to avoidmode-coupling.

[0130] In general, high levels of mode-coupling can be expected fromfibers with high scattering loss. This suggests the possibility of lowmode-coupling coefficients at long wavelengths in fibers with lowscattering loss. As can be seen from Table 1, a dramatic reduction ofmode-coupling occurs with increased wavelength in fiber 1. An acceptablelevel of mode-coupling is achieved in fiber 1 down to wavelengths asshort as 790 nm. Since the number of modes of an optical fiber dependsonly on the ratio a/λ, a fiber similar to fiber 1 with a core diameteras large as 56 μm can produce acceptable levels of mode-coupling in a 1m length. Due to the reduction of scattering at longer wavelengths, evenlarger core diameters are acceptable at longer wavelengths. For example,a MM fiber with a core diameter of 60 μm can amplify pulses with a peakpower 16 times larger than possible with SM amplifiers described byTaverner et al. Indeed, acceptable levels of mode coupling were obtainedfor a specifically designed fiber with a 50 μm core diameter as evidentfrom Table 1 and explained in the following.

[0131] Further, it is clear that, to minimize mode-coupling, step-indexMM fibers are more useful than graded-index MM fibers, since thepropagation constants in graded-index fibers are very similar, whichgreatly increases their sensitivity to mode coupling. To minimizemode-coupling, the difference in the propagation constants between fibermodes is preferably maximized.

[0132] Fiber 2 was manufactured by a rod-in-tube technique withintrinsic high scattering losses leading to much larger mode-couplingcoefficients compared to the MCVD-grown fibers 1, 3 and 4. Also, themode-coupling coefficients measured in fiber 2 are similar to resultsobtained by Gambling et al. and Griebner et al., who also usedstep-index solid-core fibers manufactured by rod-in-tube techniques. Asa consequence, reduced mode-coupling can be expected from directly grownMM fibers employing, for example, MCVD, OVD, PCVD or VAD fiberfabrication techniques.

[0133] As shown in Table 1, the mode-coupling coefficients obtained infiber 4 at 1.55 μm are about a factor of 11 smaller than in fiber 3.This difference is explained by the fact that the outside diameter offiber 4 is 250 μm, whereas the outside diameter of fiber 3 is 125 μm. Ingeneral, a thicker fiber is stiffer and less sensitive to bend andmicro-bend induced mode-coupling, as evident from Table 1.

[0134] In experiments conducted by the inventors, the lowestmode-coupling coefficients were obtained by longitudinally stretchingthe optical fibers. For example, the mode-scattering coefficients offiber 2 and 3 were measured while keeping the fiber under tension andwhile keeping the fiber straight. The application of tension in shortlengths of fibers can be useful in obtaining the best possiblemode-quality.

[0135] Mode-coupling was also measured in a configuration where theamplifier fiber (fiber 1) was pumped, as shown in FIG. 12. Specifically,the amplifier was pumped at a wave-length of 980 nm contra-directionallywith respect to the signal with a launched power up to 3 W from abroad-stripe semiconductor laser with an active area of 1×500 μm, wheredemagnification was employed to optimize the power coupling into theinner cladding of the MM amplifier fiber. The amplifier was cleaved atan angle of about 8° to eliminate spurious feedback. A signal power upto 100 mW was then extracted from the amplifier system at 1.56 μm.

[0136] The coupling efficiency of the MM amplifier fiber 1012 into theMF fiber 1016 as a function of bend-radius of the MM amplifier fiber1012 is shown in FIG. 13. For a straight MM amplifier fiber and for abend-radius of 10 cm, a coupling efficiency up to 94% is obtained intothe MF fiber 1016, demonstrating that mode-coupling is nearly completelyabsent in the MM amplifier fiber 1012 and that a SM can indeed propagateover lengths of several meters in such fibers. No clear onset ofmode-coupling is visible even for a bend-radius of 5 cm, since, even inthis case, a coupling efficiency of about 90% from the MM amplifierfiber 1012 to the MF fiber 1016 is obtained.

[0137] Since the measured coupling efficiencies from the MM amplifier1012 to a SM fiber are nearly the same under unpumped and pumpedconditions, it is evident that gain-guiding is relatively weak in thisparticular amplifier fiber. This observation was also verified by asimple computer model (see below). However, clearly any dopantconfinement in the center of the MM amplifier core will predominantlylead to amplification of the fundamental mode. Any light scattered intohigher-order modes will experience less gain and, due to the reducedintensity overlap of the higher-order modes with the fundamental mode,low levels of scattered light in higher-order modes will also notsaturate the gain of the fundamental mode. Thus, while in theabove-described experimental example, the mode-scattering coefficientswere so low that any effects due to gain-guiding were not readilyobservable, in general, gain-guiding plays a role in a MM amplifiersystem according to the present invention. In addition, theabove-mentioned computer model predicts the onset of gain-guiding of thefundamental mode in MM fibers with larger core diameter and/or reducedrefractive index differences between the core and cladding.

[0138] As the mode diameter increases, the size of the SM can bedetermined by the gain profile under small signal conditions, i.e. inthe absence of gain saturation. This allows a length-dependent modesize. Initially, under small signal conditions, the mode is confined bygain-guiding. As the gain saturates, gain guiding becomes less relevantand the mode size can increase, limited eventually by the core of the MMfiber. A length-dependent mode size can also be achieved by employing acore size which tapers along the fiber length. This can, for example, beachieved by tapering the outside fiber diameter along the fiber length.

[0139] In the presence of gain-guiding, amplified spontaneous emission(ASE) is reduced, as the MM fiber essentially becomes SM. In thepresence of gain-guiding, ASE is also guided predominantly in thefundamental mode, rather than in all possible modes of the MM fiber,leading to an improvement in the noise properties of the MM fiber.

[0140] Equally, in the experimental example, dopant-confinement wasobserved to lead to a significant reduction in the amplified spontaneousemission (ASE) levels in the fiber. This was verified by measuring thecoupling efficiency of the ASE from the MM amplifier 1012 into the MFfiber 1016. In this case, no signal light was coupled into the MMamplifier fiber 1012. For an ASE power level of 1 mW, a couplingefficiency as high as 15% was measured. A comparison with equation (2)indicates that ASE is generated mainly in about 13 low-order modes (herea factor of two from polarization degeneracy is accounted for), i.e.,ASE is generated in only about 20% of the total mode-volume of theamplifier fiber. The large reduction in ASE which was observed not onlyreduces the noise level in the amplifier; low levels of ASE also allow areduction of the signal power that is required to saturate theamplifier. To extract the highest energy from an oscillator-amplifiersignal pulse source, an operation of the amplifier in saturation isgenerally preferred.

[0141] The coupling efficiency at 1.55 μm and at 780 nm from the MMamplifier fiber 1012 to the MF fiber 1016 was not found to vary whenapplying small mechanical perturbations to the optical fiber. In apractical optical system, the applied mechanical perturbations are smallcompared to the perturbations inflicted by a 5 cm bend radius, whichindicates that long-term stability of the mode-propagation pattern insuch fibers can be achieved.

[0142] The MM amplifier 1012 is polarization preserving for bend-radiias small as 10 cm. To obtain a high-degree of polarization holding,elliptical fiber cores or thermal stresses can be used in such fibers.

[0143] The autocorrelation of the amplified pulses from the MM amplifierfiber 1012 (bend radius=10 cm) measured under the condition of optimummode-match and a condition of non-optimum mode-match are respectivelyshown in FIGS. 14 and 15. Under non-optimum mode-match, theautocorrelation displays several peaks due to the excitation ofhigher-order modes, which have different propagation constants. However,under optimum mode-matching conditions, any secondary peaks aresuppressed to better than 1%, which indicates the high-quality of thepulses emerging from the MM amplifier fiber.

[0144] In general, the spectrum of the pulses measured at the output ofthe MM amplifier fiber 1012 is more critically dependent on the couplingconditions than the autocorrelation. The reason for this is that thespectral measurement is sensitive to the phase between the fundamentalmode and the higher-order modes, i.e., an energy content of higher-ordermodes of only 1% in the output of the MM fiber leads to a perturbationof the shape of the spectrum by 10%.

[0145]FIG. 16 is a block diagram of a multi-mode fiber amplifier systemaccording to a second embodiment of the present invention. The systemincludes a near-diffraction limited input beam, a mode-converter 1050and a MM fiber amplifier 1052. The near-diffraction limited input beamcan be generated from any laser system, which need not be a fiber laser.The near-diffraction limited input beam can contain cw or pulsedradiation. The mode-converter 1050 can consist of any type of opticalimaging system capable of matching the mode of the MM amplifier 1052.For example, a lens system may be employed. Alternatively, a section oftapered fiber may be employed, such that the output mode at the end ofthe tapered fiber is matched to the mode of the MM amplifier fiber 1052.In this case, the mode-converter can be spliced directly to the MM fiber1052 producing a very compact set-up. Any pumping configuration could beemployed for the MM amplifier fiber, such as contra- or co-directionalpumping with respect to the signal or side-pumping. Equally, the NA ofthe pump light could be reduced to minimize ASE. In this case, the useof just a single-clad fiber is more advantageous, where the pump lightis directed into the fiber core. In general, the MM amplifier 1052 canhave a single, double or multiple cladding.

[0146] In the case of co-directional pumping, the pump light and thesignal light are launched via a dichroic beamsplitter (not shown). Thecoupling optics are then optimized to simultaneously optimize thecoupling of the pump beam and the signal beam.

[0147] A single or a double pass of the signal through the MM fiber 1052is most convenient. In the case of a double-pass configuration, aFaraday rotator mirror can be employed to eliminate polarization driftsin the system. Of course, in a double-pass configuration, after thefirst pass through the amplifier the coupling of the signal intohigher-order modes must be avoided to ensure a near-diffraction limitedoutput.

[0148] Optionally, linear or nonlinear optical elements can be used atthe output of the system. Such a system is compatible with anyapplication that has been used in conjunction with conventional lasersystems.

[0149] Many nonlinear applications indeed require high peak pulse powersfor their efficient operation, which are very difficult to achieve incladding-pumping SM amplifiers due to the 10 s of meters of fiber lengththat are typically employed in such systems. Even in standard SM opticalamplifiers, peak powers greater than 1 kW/amplifier length can rarely beachieved. In contrast, peak powers of ≈15 kW are achievable in a 1.5 mlength of double-clad Er/Yb fiber (fiber 1 from Table 1) withoutappreciable non-linear effects, i.e., peak powers greater than 20kW/amplifier length can be achieved.

[0150] According to the present invention, the use of a MM amplifier isbeneficial not only by way of allowing the use of a large core diameter;the use of a MM amplifier also allows a reduction of the ratiocladding/doped core diameter, which minimizes the amplifier length andthus the amplifier non-linearity. However, this leads to the generationof more ASE noise.

[0151]FIG. 17 is a block diagram illustrating a multi-mode fiberamplifier system according to a third embodiment of the presentinvention. In the system of the third embodiment, high-power opticalpulses can be propagated (or amplified) in undoped (or amplifier) MMfibers, such that spectral broadening is obtained to allow for pulsecompression of the amplifier output. For applications in nonlinearpulse-compression, optical fibers with either positive(non-soliton-supporting) or negative (soliton-supporting) dispersion canbe employed. The power levels in the multi-mode fiber 1060 are raised toobtain an appreciable amount of self-phase modulation. The interplay ofdispersion and self-phase modulation in the optical fiber can then beused to broaden the spectrum of the optical pulses and to obtain pulsecompression.

[0152] When the MM fiber 1060 is soliton supporting, higher-ordersoliton compression may be used to produce short pulses from the MMfiber 1060 directly. In general, in the case of positive dispersion(non-soliton supporting) fiber, additional linear or nonlinearpulse-compression components must be used to compress the spectrallybroadened optical pulses. In this case, a conventional linear pulsecompressor 1062 (such as a prism, grating, grism or SM chirped fiberBragg grating) may be used at the output of the system. Also, chirpedperiodically poled doubling crystals may be used to obtain a compressed,frequency-doubled pulse. Equally, chirped fiber Bragg gratings may bewritten into the MM optical fiber 1060 with reduced mode-coupling toreduce the nonlinearities of such structures when applied to linearpulse compressor 1062. The Bragg grating should not be blazed toeliminate the excitation of higher-order modes in reflection.

[0153]FIG. 18 is a diagrammatic view of a system according to a fourthembodiment of the present invention. As shown in FIG. 18, a mode-filter1070 is inserted in front of one of the cavity mirrors M1 and M2 toensure a diffraction-limited output of the system. The mode filter 1070can consist of a standard SM fiber in conjunction with appropriatemode-matching optics. Alternatively, a tapered fiber can be used (asdiscussed above) to provide for mode-matching. For optimum mode-couplingthe efficiency of the laser will be nearly as high as for an all-SMlaser. However, the use of MM amplifier 1076 allows for increased designflexibility. Thus, double-clad erbium/ytterbium fibers with differentcore-cladding ratios can be employed wherever appropriate.

[0154] According to a fifth embodiment, the use of MM fiber allows thedesign of double-clad fibers with low absorption cross sections. Forexample, a double-clad Er-doped amplifier fiber may be constructed fromMM fibers. Typically Er-doped double-clad fibers are relativelyinefficient, since large cladding/core ratios have to be employed inorder to absorb pump light from broad area diode lasers while stillpreserving a SM fiber core. Typically, such a design would involve aΦ_(cl)=100 μm diameter cladding and a Φ_(co)=10 μm diameter core. Theeffective absorption of such a structure is 100 times (=Φ_(cl)/Φ_(co))²smaller than the absorption in a single-clad Er-doped fiber. Thus, 100times longer fiber amplifier lengths are required in this case. However,by implementing MM Er-doped fiber, the core size can be greatlyincreased, producing much smaller cladding/core ratios and shorteramplifier lengths which is very beneficial for the design of high-powerlasers. Of course, for the design of high-power Er double-clad lasers,cladding diameters even larger than 100 μm can be implemented. Aconceptual drawing of a fiber cross section employing a doped MM fibercore and an undoped fiber cladding is shown in FIG. 19. As shown in FIG.19, the active dopant is confined in a cross section, defined by thedopant profile, substantially smaller than the fiber core, as defined bythe refractive index profile. Of course, in such laser system, dopantconfinement increases the amplifier length, thus only relatively weakdoping confinement is useful.

[0155] According to a sixth embodiment of the present invention, asshown n FIG. 20, a fiber regenerative amplifier may be constructed froma MM fiber amplifier 1090. A regenerative amplifier is useful forobtaining mJ energies from MM fiber amplifiers. Due to the limited gainof MM fiber amplifiers, the extraction of mJ energies will typicallyrequire several passes through the amplifier, which is facilitated bythe regenerative amplifier. As shown in FIG. 20, a fast optical switch(OS) 1092 is used to switch the pulses in and out of the regenerativeamplifier. A mode-filter 1094 can also be included to “clean-up” thefiber mode in the amplification process. The mode-filter 1094 canconsist of a spatial filter to minimize any nonlinearities in theregenerative amplifier.

[0156] The seed pulse is selected from the oscillator 1096 by theoptical switch 1092 at the desired repetition rate. The Faraday rotator1098 and the polarization beam splitter 1099 are used to couple theamplified pulse out of the system.

[0157] Either cw or pulsed pumping of the amplifier can be employed.

[0158] According to a seventh embodiment of the present invention shownin FIG. 21, a MM Q-switched fiber laser source is constructed. The largecross-sections possible with MM fibers allow greatly increasing theenergy storage compared to a single-mode fiber. As a result, high-powerQ-switched pulses may be directly generated from such a system.Typically, these pulses have a duration in the nsec regime. As shown inFIG. 21, a mode-filter 10100 can also be included to ensure an optimummode-quality. The optical switch 10102 is employed for output couplingand it also serves to modulate the loss (Q) of the cavity defined by thetwo mirrors M1 and M2 and the MM amplifier 10104. Alternatively, theoutput can be extracted by using a partially transmissive mirror M2.

[0159] According to an eighth embodiment of the present invention shownin FIG. 22, a preamplifier is included in front of the final MMamplifier fiber 10112 to fully saturate the MM amplifier fiber 10112 andto reduce the level of ASE in the MM amplifier fiber 10112. Thepreamplifier can be SM and also MM, where it is useful to select thecore radius of the preamplifier fiber 10110 to be smaller than the coreradius of the final MM amplifier fiber 10112 to minimize the growth ofASE. One isolator (not shown) can be inserted between the laser sourceand the preamplifier and another isolator (not shown) can be insertedbetween the preamplifier 10110 and the final MM amplifier fiber 10112further to reduce ASE. Similarly, narrow band optical filters (notshown) can be included anywhere in the system to reduce ASE. Also,optical switches (not shown) can be used in between the laser source,the preamplifier 10110 and the final amplifier 10112 to reduce theamount of ASE.

[0160] More than one preamplifier can be used in the system, whereisolators and optical filters and optical switches can be used tominimize the amount of generated ASE in the system. Further, nonlinearprocesses in the preamplifiers and the final MM amplifier can be usedfor pulse compression.

[0161] According to a ninth embodiment of the present invention shown inFIG. 23, a frequency converter 10120 is included downstream of the MMamplifier fiber 10122 to frequency convert the output amplified beam.The frequency converter can be a non-linear crystal, such as aperiodically-poled or aperiodically poled LiNbO₃ crystal which frequencydoubles the output beam.

[0162] Although several exemplary embodiments have been herein shown anddescribed, those of skill in the art will recognize that manymodifications and variations are possible without departing from thespirit and scope of the invention, and it is intended to measure theinvention only by the appended claims.

What is claimed is:
 1. A laser for generating ultra-short opticalpulses, comprising: a cavity which repeatedly passes light energy alonga cavity axis; a length of multi-mode optical fiber doped with a gainmedium and positioned along said cavity axis; a pump for exciting saidgain medium; a mode locking mechanism positioned on said cavity axis;and an optical guide positioned on said cavity axis which confines thelight amplified by said multi-mode optical fiber to preferentially thefundamental mode of said multi-mode optical fiber.
 2. A laser forgenerating ultra-short optical pulses as defined in claim 1 wherein saidmode locking mechanism comprises a passive mode locking element.
 3. Alaser for generating ultra-short optical pulses as defined in claim 2wherein said passive mode locking element comprises a saturableabsorber.
 4. A laser for generating ultra-short optical pulses asdefined in claim 3 wherein said saturable absorber comprises InGaAsP. 5.A laser for generating ultra-short optical pulses as defined in claim 3additionally comprising a power limiter for protecting said saturableabsorber.
 6. A laser for generating ultra-short optical pulses asdefined in claim 5 wherein said power limiter comprises a two photonabsorber.
 7. A laser for generating ultra-short optical pulses asdefined in claim 1 wherein said optical guide comprises a single-modemode-filter fiber on said cavity axis.
 8. A laser for generatingultra-short optical pulses as defined in claim 7 wherein saidsingle-mode mode-filter fiber is fusion spliced onto one end of saidmulti-mode optical fiber.
 9. A laser for generating ultra-short opticalpulses as defined in claim 8 wherein said multi-mode fiber is tapered atsaid fusion splice.
 10. A laser for generating ultra-short opticalpulses as defined in claim 8 wherein said single-mode mode-filter fiberis tapered at said fusion splice.
 11. A laser for generating ultra-shortoptical pulses as defined in claim 8 wherein both said single-modemode-filter fiber and said multi-mode fiber are tapered at said fusionsplice.
 12. A laser for generating ultra-short optical pulses as definedin claim 1 wherein said pump is coupled to said multi-mode fiber alongsaid cavity axis.
 13. A laser for generating ultra-short optical pulsesas defined in claim 1 wherein said pump is coupled to the side of saidmulti-mode fiber.
 14. A laser for generating ultra-short optical pulsesas defined in claim 13 additionally comprising an optical coupler forcoupling said pump to said multi-mode fiber.
 15. A laser for generatingultra-short optical pulses as defined in claim 13 additionallycomprising a v-groove on said multi-mode optical fiber for coupling saidpump to said multi-mode fiber.
 16. A laser for generating ultra-shortoptical pulses as defined in claim 1 additionally comprising apolarization beam splitter for outputting said ultra-short opticalpulses from said laser.
 17. A laser for generating ultra-short opticalpulses as defined in claim 1 wherein said cavity comprises a pair ofreflectors at its opposite ends.
 18. A laser for generating ultra-shortoptical pulses as defined in claim 17 wherein one of said pair ofreflectors is partially reflecting and provides the output for saidcavity.
 19. A laser for generating ultra-short optical pulses as definedin claim 17 wherein said mode locking mechanism comprises a saturableabsorber, and wherein one of said reflectors is formed on a surface ofsaid saturable absorber.
 20. A laser for generating ultra-short opticalpulses as defined in claim 19 wherein said mode locking mechanismadditionally comprises a power limiter for protecting said saturableabsorber, and wherein said saturable absorber is formed on a surface ofsaid power limiter opposite said one of said reflectors.
 21. A laser forgenerating ultra-short optical pulses as defined in claim 20 whereinsaid power limiter comprises a two-photon absorber.
 22. A laser forgenerating ultra-short optical pulses as defined in claim 1 additionallycomprising a linear phase drift compensator on said cavity axis.
 23. Alaser for generating ultra-short optical pulses as defined in claim 22wherein said linear phase drift compensator comprises a Faraday rotator.24. A laser for generating ultra-short optical pulses as defined inclaim 23 wherein said linear phase drift compensator comprises a pair ofFaraday rotators.
 25. A laser for generating ultra-short optical pulsesas defined in claim 22 additionally comprising a linear polarizationtransformer on said cavity axis.
 26. A laser for generating ultra-shortoptical pulses as defined in claim 25 wherein said linear polarizationtransformer comprises a wave plate.
 27. A laser for generatingultra-short optical pulses as defined in claim 1 wherein said modelocking mechanism comprises an active mode locking element.
 28. A laserfor generating ultra-short optical pulses as defined in claim 27 whereinsaid active mode locking element comprises an optical amplitudemodulator.
 29. A laser for generating ultra-short optical pulses asdefined in claim 27 wherein said active mode locking element comprisesan optical frequency modulator.
 30. A laser for generating ultra-shortoptical pulses as defined in claim 1 wherein said ultra-short opticalpulses preferentially in the fundamental mode of said multi-mode opticalfiber have a pulse width below 500 psec.
 31. A laser for generatingultra-short optical pulses as defined in claim 1 additionally comprisingan environmental stabilizer on said cavity axis to assure that saidcavity remains environmentally stable.
 32. A laser for generatingultra-short optical pulses as defined in claim 31 wherein saidenvironmental stabilizer comprises a Faraday rotator.
 33. A laser forgenerating ultra-short optical pulses as defined in claim 32 whereinsaid environmental stabilizer comprises a pair of Faraday rotators. 34.A laser for generating ultra-short optical pulses as defined in claim 1wherein said optical guide comprises an optical fiber doped with anamplifying medium to provide gain guiding.
 35. A laser for generatingultra-short optical pulses as defined in claim 34 wherein saidamplifying medium is concentrated centrally within a fraction of thecore diameter of said optical fiber.
 36. A laser for generatingultra-short optical pulses as defined in claim 1 wherein said opticalguide comprises a single-mode optical fiber on said cavity axis.
 37. Alaser for generating ultra-short optical pulses as defined in claim 1wherein said optical guide comprises a mode-filter on said cavity axis.38. A laser for generating ultra-short optical pulses as defined inclaim 37 wherein said mode filter excites the fundamental mode of saidmulti-mode fiber.
 39. A laser for generating ultra-short optical pulsesas defined in claim 38 wherein said mode filter excites the fundamentalmode of said multi-mode fiber with an efficiency of at least 90%.
 40. Alaser for generating ultra-short optical pulses as defined in claim 1wherein said cavity additionally comprises a positive dispersionelement.
 41. A laser for generating ultra-short optical pulses asdefined in claim 40 wherein said positive dispersion element comprises alength of single-mode positive dispersion fiber positioned along saidcavity axis.
 42. A laser for generating ultra-short optical pulses asdefined in claim 41 additionally comprising an output coupler forlimiting the light energy at said single-mode positive dispersion fiberto less than 10% of the peak power in said cavity.
 43. A laser forgenerating ultra-short optical pulses as defined in claim 42additionally comprising a frequency converter for compressing pulsesgenerated by said cavity.
 44. A laser for generating ultra-short opticalpulses as defined in claim 43 wherein said frequency converter comprisesa frequency doubler.
 45. A laser for generating ultra-short opticalpulses as defined in claim 44 wherein said frequency doubler compriseschirped periodically poled LiNbO3.
 46. A laser for generatingultra-short optical pulses as defined in claim 1 wherein said multi-modefiber includes a core, and wherein said gain medium in said multi-modeoptical fiber is concentrated centrally within the core of saidmulti-mode fiber.
 47. A laser for generating ultra-short optical pulsesas defined in claim 1 wherein said multi-mode optical fiber ispolarization-maintaining.
 48. A laser for generating ultra-short opticalpulses as defined in claim 47 wherein said polarization-maintainingmulti-mode fiber has an elliptical core.
 49. A laser for generatingultra-short optical pulses as defined in claim 47 wherein saidpolarization maintaining multi-mode fiber comprises stress-producingregions.
 50. A laser for generating ultra-short optical pulses asdefined in claim 1 wherein said cavity additionally comprises a fibergrating written onto said multi-mode fiber, said grating primarilyreflecting the fundamental mode of said multi-mode fiber.
 51. A methodof generating ultra-short pulses, comprising: providing a length ofoptical fiber doped with a gain medium; repeatedly passing signal lightthrough said length of optical fiber to produce said ultra-short pulses;and providing sufficient stored energy within said gain medium toamplify said pulses to a peak power above 1 KW.
 52. A method ofgenerating ultra-short pulses as defined in claim 51 additionallycomprising: environmentally stabilizing said optical fiber.
 53. A methodof generating ultra-short pulses as defined in claim 51 additionallycomprising mode-locking said optical fiber.
 54. A method of generatingultra-short pulses as defined in claim 51 wherein said providing stepcomprises providing a multi-mode fiber doped with a gain medium.
 55. Amethod of generating ultra-short optical pulses, comprising: circulatinglight energy within a cavity; amplifying said light energy within saidcavity in a multi-mode fiber; and confining said light energy withinsaid cavity substantially to the fundamental mode of said multi-modefiber.
 56. A method of generating ultra-short optical pulses as definedin claim 55 additionally comprising mode locking said light energy. 57.A method of generating ultra-short optical pulses as defined in claim 55wherein said confining comprises mode filtering said light energy.
 58. Amode-locked laser for generating high power ultra-short optical pulses,comprising: a multi-mode optical fiber doped with gain material foramplifying optical energy; means for pumping said optical fiber; andmeans for confining the optical energy amplified by said multi-modeoptical fiber to substantially the fundamental mode of said multi-modeoptical fiber.